Plants & Ecology
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Comparing avifauna communities and bird functional diversity of forest and farmland in southwest Ethiopia by Dries Engelen Plants & Ecology Plant Ecology 2012/5 Department of Botany Stockholm University Comparing avifauna communities and bird functional diversity of forest and farmland in southwest Ethiopia by Dries Engelen Supervisor: Kristoffer Hylander Plants & Ecology Plant Ecology 2012/5 Department of Botany Stockholm University Plants & Ecology Plant Ecology Department of Botany Stockholm University S-106 91 Stockholm Sweden © Plant Ecology ISSN 1651-9248 Printed by FMV Printcenter Cover: Middle photo: Black-crowned Cranes in the farmland. Other pictures from top left to bottom right: Four forest specialist birds: Zoothera Piaggiae, Mandingoa nitidula, Oriolus monacha & Columba larvata. Photos by Dries Engelen. ABSTRACT - Worldwide degradation and conversion of tropical forests affects many species and their provided ecosystem services. Among them are birds, responsible for pollination, seed dispersal, pest control and scavenging. This study, conducted in southwest Ethiopia, compares species composition and bird functional diversity between forest and homegardens close to and far from forest, both in terms of species numbers and bird abundances. Point counts and mist netting were used to obtain data. While the former method detected more species, abundance data from the latter revealed patterns not observed by just comparing species numbers. I found that species diversity was lower in forest compared to gardens and that the species composition of both communities was significantly distinct. Whereas forest had more forest specialists, gardens held more forest visitors and species of open country. Close and far gardens did not differ in any aspect, except that abundances of forest generalist birds were somewhat higher close to forest. Regarding bird feeding guilds, I found that granivores and nectarivores were more numerous in gardens, while frugivores were more common in forest. Carnivores and omnivores showed no effect. Insectivore proportions were the same for forest and farmland, but their numbers (including those of all sub-guilds) were higher in gardens. The Ethiopian forest avifauna is poor in comparison with other Afromontane regions, lacking several, mostly insectivorous genera. This could be the result of an extinction after which its geographic isolation made recolonization difficult, especially for dispersal-limited understory species. Nonetheless, and despite their impoverished state, the results suggest that forest remnants are important for forest-dependent species, being a stronghold for forest specialists and supporting higher numbers of forest generalists in nearby gardens. However, future forest regrowth might be at stake with ongoing agricultural encroachment, because gardens hold fewer frugivores, especially forest specialists, which might affect seed dispersal. SAMMANFATTNING - Många arter påverkas negativt av avskogning och annan mänsklig påverkan på tropiska skogar, vilket också kan få konsekvenser för de ekosystemtjänster dessa arter levererar. En viktig artgrupp i detta avseende är fåglar, som kan ha betydelse för pollination, fröspridning, naturlig skadedjurskontroll och som asätare. Denna studie, utförd i sydvästra Etiopien, jämför artsammansättning och funktionell diversitet hos fåglar i skogar och trädgårdar både nära och långt ifrån skogen. Både antal arter och antal individer har undersökts. Datamaterialet bygger både på observationer från punktinventering och fåglar fångade med slöjnät. Med den första metoden noterade jag fler arter, medan abundansvärden från den andra metoden gjorde att jag såg mönster som inte syntes vid endast jämförelser av artrikedom. Jag fann att artrikedomen var lägre i skogen i jämförelse med trädgårdarna, men att artsammansättningen var signifikant skiljd däremellan. Skogarna hade fler skogsspecialister medan trädgårdarna hade fler arter från öppna marker och tillfälliga skogsbesökande arter. Trädgårdar som låg nära eller långt ifrån skogen skiljde sig inte på något sätt utom att individtätheten av skogsgeneralister var något högre i trädgårdar nära skogen. När det gäller uppdelningen av fåglarna utifrån vad de äter så fann jag att fröätande och nektarätande arter var vanligare i trädgårdar medan fruktätande arter var vanligare i skogen. Rovfåglar och allätare uppvisade inget tydligt mönster. Proportionen insektsätande fåglelarter var samma i skogen och trädgårdarna, men det absoluta antalet arter (inklusive alla olika underkategorier) var högre i trädgårdarna. Den etiopiska skogsfågelfaunan är artfattig i förhållande till andra bergstrakter i Afrika och saknar flera släkten av framförallt insektsätande fåglar. Detta skulle kunna bero på ett tidigare utdöende varefter en senare återkolonisering varit svår på grund av den geografiska isoleringen, speciellt för insektsätande fåglar som huvudsakligen finns i undervegetationen. Trots den relativa artfattigdomen så visar resultatet från min studie hursomhelst att skogarna är viktiga för skogsberoende fåglar. Den största betydelsen har de för skogsspecialister där, men även genom en positiv effekt på abundansen av skogsgeneralister i närliggande trädgårdar. Dock kan framtida återväxt av skogar bli problematisk på grund av en pågående omvandling av skogar till trädgårdar, eftersom trädgårdar hyser färre fruktätande fåglar, vilket kan påverka fröspridning över landskapet. KEYWORDS: bird diversity, tropical agroecosystem, Ethiopia, feeding guilds, distance to forest, mist netting, point count. 3 TABLE OF CONTENTS Abstract & Sammanfattning ………………………………………………………………….. 3 Table of Contents …………………………………………………………………………….. 4 Introduction ………………………………………………………….……………………….. 5 Material & Methods ………………………………………………………………………….. 9 Study area & sites …………………………………………………………………….. 9 Bird surveys & classifications …..…………………………………….…………….. 10 Data analyses ….…………………………………………………………………….. 12 Results ………………………………………………………………………………………. 14 General patterns in the bird community ……...……………………………………... 14 Feeding guilds ………………………………………………………………………. 17 Mist netting vs. point counts ………………………………………………………... 18 Discussion …………………………………………………………………………………... 20 Species composition of forest and farmland ………………………………………... 20 Effect of distance to forest ………………………………………………………….. 22 Feeding guild compositions ………………………………………………………… 23 Comparing bird survey techniques …………………………………………………. 26 Conclusions ………………………………………………………………………………… 28 Acknowledgements ………………………………………………………………………… 29 References ………………………………………………………………………………….. 30 Appendix …………………………………………………………………………………… 34 4 INTRODUCTION Habitat degradation and severe ecosystem alterations due to anthropogenic activities is the most important cause for biodiversity losses worldwide (Vitousek et al. 1997). Tropical regions, which harbor the vast majority of this diversity (Gray et al. 2007), are subjected to increasing land-cover changes as a result of accelerating human population growth (Teketay 1992). Worldwide, tropical forests are being logged and degraded because of an increasing demand for forest resources, or are converted into farmland and plantations (Laube et al. 2008). Because degraded and modified habitats make up a growing proportion of the tropics nowadays, it is important to assess their ability to sustain biodiversity (Gray et al. 2007; Laube et al. 2008). Alterations in species richness and composition can also affect the functional diversity of the community (Gray et al. 2007) and changes in provided ecosystem services can, in turn, have an effect on humans again (Clough et al. 2009). One group responsible for a number of ecosystem services is birds, which play an important role in pollination, pest control, seed dispersal and scavenging. A decline in their numbers would therefore mean a decline in their provided services (Şekercioğlu et al. 2004). Of course changes in bird composition may also affect their ecosystem disservices, such as seed-eating. Synthesizing the results of 57 studies from Asia and the Neotropics on tropical birds specifically, Gray et al. (2007) found that birds from different feeding guilds respond differently to forest disturbance. Whereas granivorous species increase significantly after disturbance, the abundance of frugivores and insectivores significantly decreased. Declines in the numbers of omnivores, carnivores and nectarivores were also observed, though less outspoken because of regional differences. In another study based on global data Tscharntke et al. (2008) found similar results for granivores and insectivores, but instead noticed an increase in (small) frugivores and nectarivores with the conversion of forests to agriculture (until a point when disturbance was so severe that also these groups declined). Furthermore results showed that birds in agricultural habitats are more often generalists. Overall the increase of granivores and the decline of insectivores and large frugivores with forest modification are most strongly supported (Sodhi et al. 2008). The negative impact on insectivores does, however, differ among the various sub-guilds (Dale et al. 2000) and seems most outspoken for species of the understory and large insectivores in general. Birds of the 5 understory are thought to be so sensitive to disturbance because of their inability to disperse in a non-forest matrix (Newmark 1991; Şekercioğlu et al. 2002). Whether the same patterns occur in Africa is hard to say since the region is relatively understudied (Gray et al. 2007; Laube